The Mid-and-Long-Term Roadmap has established the first half of FY2018 to be the point at which the method of fuel debris removal must be decided upon by the NDF.

Fuel debris needs to be retrieved with proven technologies after careful preparations and stored in a stable condition to reduce risks. This should proceed as follows: (1) maintaining and managing fuel debris in a stable condition until it is retrieved; (2) safe retrieval of the fuel debris; and (3) storage of the retrieved fuel debris in a stable condition after being collected and transported.

At present, conceptual investigations and studies are being carried out, as are feasibility studies (FS) and element tests to a certain extent. After the retrieval method is selected, basic/detailed design for the initial unit and practical application and verification testing of equipment and devices will take place towards application to the actual unit.

The fuel retrieval plan is to be established after confirming the conditions of fuel debris.

The location of fuel debris in each unit is to be estimated based on a comprehensive evaluation leveraging such methods as observing the inside of the PCV with cameras, simulating reactor core conditions using safety analysis code based on changes in pressure and temperature at the time of accident, checking the location of fuel debris with muon detection technology, and current dose and temperature distribution through investigations using robots.

In addition, plans for PCV internal investigations will be established based on the analyses of fuel debris from the 1979 TMI-2 accident and simulated debris to make assumptions on the location and properties of fuel debris.

*Muon detection technology:
Technology used to identify the location and shape of fuel by using the characteristics of cosmic ray mu particles (muons) from space and the atmosphere, the numbers and path of which varies depending on the density of the materials they pass through.

2.3. Fuel debris retrieval (Four different water levels and three access directions)

In order to establish a fuel debris retrieval method for the Fukushima Daiichi NPS, studies have been conducted that employ the retrieval method used at TMI-2, where fuel debris was submerged in water in order to reduce dose rate utilizing the so-called “submersion method.”

However, there are many challenging issues involved in the development of technologies required to repair the PCV damaged due to the severity of the accident in order for it to be filled with water. Assuming it would be a challenge for fuel debris to be submerged in its entirety, it is also necessary to seek methods of retrieving fuel debris while it is exposed to the air and without filling the PCV with water to the top.

Full submersion method：

A method of retrieving fuel debris in a submerged state by filling the PCV with water to the top. While this method is an excellent way of keeping fuel debris cool, shielding from radiation, and preventing the dispersion of radioactive dust, problem areas exist, such as preventing water leakage from the PCV, seismic resistance, and radiation shielding.

Partial submersion/Dry method:

A method to retrieve fuel debris exposed in the air without filling the PCV with water. Entry from both the top and the side is possible using this method, but there are some difficulties in preventing the dispersion of radioactive dust and in radioactive shielding.

Water level in the PCV (Four types)

●Full submersion method: Filling the reactor well to the top with water.
●Submersion method: Filling the PCV with water to a level above the highest point of the distributed fuel debris.
●Partial submersion method: Filling the PCV with water to a level below the highest point of the distributed fuel debris.
●Dry method: All areas where the fuel debris is distributed are exposed in the air, and neither cooling nor filling the PCV with water is involved.

Fig. 4.3.2-2 Method classification according to PCV water level

Access directions to fuel debris (Three types)

●Access from the top of the PCV (Top entry)
●Access from the side of the PCV (Side entry)
●Access from the bottom of the PCV (Bottom entry)

Fig. 4.3.2-3 Access directions to the fuel debris

Method to be focused on

As described above, there are 12 possible patterns of fuel debris retrieval by combining different PCV water levels and the access directions. The three methods below have been determined to be those that should be focused on.

2.4. Studies on multiple scenarios based on the condition of each unit

This section describes fuel debris retrieval scenarios (process order and method depending on the location of the fuel debris) to be applied in each unit based on their present status.

Seven scenarios

There are a total of seven scenarios being considered for fuel debris retrieval method, including three scenarios assuming the application of (1) Submersion-Top entry method, (2) Partial submersion- Top entry method, and (3) Partial submersion-Side entry method for the entire process from the start to the completion of the retrieval work, and four scenarios applying two methods depending on the distribution of fuel debris and the order of the construction process.

Table. 4.3.5-1 Procedures and features of each scenario

Scenario selection workflow

As shown on the left side of the chart below, the selection of fuel retrieval scenarios is carried out by conducting initial investigations, understanding conditions, evaluating retrieval method feasibility, and then evaluating the scenario.

Fig. 4.3.5-9 Fuel debris retrieval scenario selection flow

Studies of the first three steps in the above process are already underway. The initial investigation stage described above has seen the use of robots to survey the inside of the PCV and of muon tracking technology to determine the location of fuel debris. In addition, analysis using accident progression analysis codes is also being carried out.

Using results obtained in the initial investigation stage, the location of fuel debris and fission products are evaluated comprehensively in order to understand conditions.

In tandem with understanding conditions, evaluation of the feasibility of the submersion and partial submersion methods is to take place. For this item, detailed studies of the feasibility of the following three methods will take place: (1) Submersion-Top entry method, (2) Partial submersion-Top entry method, and (3) Partial submersion-Side entry method).

Related information will eventually be compiled for scenario evaluation, and practical scenarios are narrowed down for each unit considering accessibility depending on the feasibility of the method and the location of fuel debris.

In addition, the results of the final scenario evaluation will be assessed based on the “The Five Guiding Principles for risk reduction” (Safe, Proven, Efficient, Timely and Field-oriented).